Air classifiers are most commonly used for separating coarse from fine particles. Most classifiers use the combined action of drag and centrifugal forces to separate materials of different diameter or density. The drag force is generated by the flow of air and the centrifugal force is produced by rotating the air flow in some manner.
Demanding applications for classification can be found in the chemical, food, ceramics, electronics, and pharmaceuticals industries, where narrow particle size distributions are desired. Many natural minerals and chemical raw materials can be used in new applications with improved economy when they are turned into fine powders. The size of the fines is determined by requirements of the product application.
A conventional rotating vane classifier consists of a cylinder with a rotor, which turns radial vanes. The vanes extend only a limited distance to the axis to avoid particle clogging. Particles are fed to the classifier near the outer radius, perpendicular to the radial air stream. When air flow passes through the vanes, it is given a tangential velocity component. Particles above the cut size are collected at the periphery of the classifier due to the centrifugal force, while those below the cut size are carried with the air to the center of the classifier due to the drag force, where they exit, as shown in FIG. 1. For larger particles, the centrifugal force is predominant because it is proportional to dp3 while the drag force is proportional to dp, where dp is the particle diameter; consequently they tend to be collected to the outside of the vanes. If the drag force acting on a particle is greater than the centrifugal force, the particle will pass through the vanes and will be collected into the fine fraction. Otherwise it will be rejected by the rotating vanes and go to the coarse fraction. The flow within the vanes where particles are separated is a forced vortex, and it will provide sharper cuts compared to other air classifiers without rotating vanes, such as cyclone separators and cross-flow air classifiers.
Rotating vane classifiers have been widely used to separate particles according to size in industries because they provide not only low cut size but also high cut sharpness. But they have a significant disadvantage in that particles are fed to the outside of the fast rotating vanes, which can cause particle attrition and deteriorate classification performance.
The cut size of particles depends on various parameters of the classifier, such as rotor shape, the number of blades, rotor speed, feed rate and airflow.